One Universe at a Time

The Last Eclipse

One thing 2017 has going for it is a total solar eclipse. Such eclipses are relatively common, but they often occur in hard to reach areas where not many people live. But the eclipse this Fall will wander across the central US, making it highly accessible. Such solar eclipses are only possible thanks to the favorable orbital geometries of the Sun, Moon and Earth, but its those same geometries that mean such total solar eclipses will eventually come to an end.

Total eclipses are only possible because the Moon has about the same apparent size as the Sun. The diameter of the Sun is about 400 times larger than the diameter of the Moon, while Moon is about 389 times closer to the Earth than the Sun. So it is possible for the Moon to completely cover the Sun when they line up the right way. It doesn’t happen every month because the Moon’s orbit is tilted a few degrees from the orbital plane of Earth, so sometimes the Moon passes a bit above or below the Sun, and casts no shadow on the Earth.

An annular eclipse showing the “ring of fire.” Credit: Kevin Baird

But even when things line up perfectly, there isn’t always a total eclipse. The Moon’s orbit around the Earth isn’t perfectly circular. Likewise, the Earth’s orbit around the Sun isn’t perfectly circular either. So the apparent sizes of the Sun and Moon vary slightly, and this means sometimes the Moon can appear slightly smaller than the Sun during a solar eclipse. When this happens, it is an annular (ring) eclipse, since a thin outer ring of the Sun can still be seen.

In our present era, both total and annular eclipses can occur. But because of the tidal forces between the Earth and Moon, the Moon is gradually moving farther away from Earth. In the distant past, the Moon was closer, so annular eclipses weren’t possible. As the Moon continues to recede from Earth, total eclipses will only occur when the Moon is at a particularly close point in its orbit (perigee), while the Sun is near its most distant (aphelion). Over millions of years, annular eclipses will become the norm, and total eclipses will become increasingly rare.

So when will the last total eclipse occur? We can’t pin down an exact date, but we can get a basic estimate. The Moon currently moves away from the Earth at a rate of 3.8 centimeters per year. In the past that rate was slower, at about 2.2 centimeters a year. If we use about 3 centimeters per year as an average, then we can simply estimate how long it will take for the Moon’s apparent size at perigee to be the same size as the Sun’s apparent size at aphelion. It comes out to be about a billion years. Of course on that time scale other factors come into play. The Sun is gradually getting hotter, and expanding slowly as a result, which would shorten the time until the last total eclipse. But the Earth is also slowly moving away from the Sun as our star radiates energy and mass. Subtle gravitational effects between the Earth and other planets can also shift Earth’s orbit slightly, as well as the Moons. All of this can come into play. But if we only want a rough estimate, we can safely say that in about a billion years the days of total eclipses will come to an end.